Making n-sided polygons in Blender 2.69
Yes This is possible.

For a cylinder:


Press ShiftA> Add mesh > Cylinder
Press F6 and set the Cap Fill Type to Ngon.


If you are looking to do this from python:

bpy.ops.mesh.primitive_cylinder_add(radius=1, depth=2, end_fill_type='NGON', view_align=False, enter_editmode=False, location=(-2.06615, -0.413393, 3.45981), layers=(True, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False, False))


Or have a look at the bmesh api.

For doing this in the interface, usually one uses Edit mode (perhaps you could elaborate on why this doesn't work for you?). To create an ngon in edit mode, select the faces you wish to convert to a single face and press F.